Formic acid, ammonium formate, and ammonium phosphate were obtained from Sigma Aldrich; LC-MS-grade methanol and acetonitrile from Burdick & Jackson; Strata-X-CW weak cation exchange (WCX) solid-phase extraction (SPE) cartridges (3 mL) from Phenomenex; and Oasis WCX [mu]Elution 96-well SPE plates from Waters.
 Nonstandard abbreviations: MRM, multiple reaction monitoring; [MRM.sup.3], MRM with multistage fragmentation; Q/LIT, quadrupole linear ion trap; WCX, weak cation exchange; SPE, solid-phase extraction; HILIC, hydrophilic interaction liquid chromatography; HMMA, 4-hydroxy-3-methoxymethamphetamine; EQA, external quality assurance; LLOQ, lower limit of quantification.
Based on reversed phase chromatography (RPC) or weak cation exchange
(WCX), the beads can fractionate mixed protein and peptide samples quickly and easily for downstream analysis such as mass spectrometry.
Finally, hepcidin concentrations for 28 plasma/serum samples obtained by the Anticalin assay were compared with values found by our previously validated weak cation exchange
time-of-flight MS assay (4) and a competitive ELISA (5) for which reference ranges have been described (2).
Oasis Weak Cation Exchange
(WCX) and Oasis Weak Anion Exchange (WAX) sorbents are sample preparation products that improve the recovery of strong acidic and basic compounds in drug metabolism, ADME and toxicology laboratories.
We used Oasis[R] weak cation exchange
10 by 1 mm SPE cartridges (Waters Corp.) for sample extraction and performed HPLC by use of an Atlantis HILIC Silica column (particle size 3 [micro]m, 2.1 mm internal diameter by 50 mm; Waters).